1. FIELD OF THE INVENTION
This invention pertains to a dynamic packing ring seal used to close the running clearance between two relatively rotating structural members, thus forming a mutually exclusive barrier to effectively isolate two incompatibly differing environments. This invention relates to such a seal used, for example to retain lubrication within a bearing space, while excluding foreign matter from the external environment.
More specifically, this invention relates to a seal displaying a capacity to maintain isolation of the two differing environments during very large relative excursions of the two structural members, as compared to relative excursions sealed by conventionally housed elastomeric packing ring seals.
2. DESCRIPTION OF THE PRIOR ART
Elastomeric packing ring seals have been extensively studied and tested and used for many years, their capabilities and limitations are generally very well known and published. The design handbooks published by the manufacturers of such rings usually carry the disclaimer that any particular seal design may be enhanced, over and above a stock handbook design, by practical experiment and testing. In practice, the final design is not likely to deviate noticeably from the original handbook design, however.
The universal standard packing ring of the industry is the well known O-ring. Although rings of other cross-sectional shapes are available, the basic rules and functions are common to all. The choice of elastomer has a pronounced influence on such factors as thermal effects, durometer hardness, and permanent set, however the effects are generally too small to effect the gland design.
In sealing the bearings of a rotary rock bit for the oil field, the most demanding heavy duty design requirements are faced, and perhaps the most revealing study of the prior art may be found.
An early effort to use an elastomeric packing ring seal in a rock bit is described by Swart, et. a., in U.S. Pat. No. 3,299,973 (issued on Jan. 24, 1967), wherein the seal is trapped between a radially extending face on a rotating rock cutting cone, and a conical surface of a supporting journal. In this application, the elastomeric body of the O-ring is held in tensile extension, being more or less stretched in response to the excursion of the cutter relative to the journal. Elastomeric rings should not be stored or used in conditions of tensile extension, if life expectancy is a criterion of judgement, especially at elevated temperatures. Such a ring in tension tends to crack and split radially. The use of an elastomeric ring in tensile extension seems also to aggravate the problem of permanent set in the elastomer.
Standard O-ring practice is to place the ring in compression, at least in the primary axis of its action in a particular design, and to provide a generous clearance in one inactive axis to provide for tolerance stack up and for thermal expansion in use. Used as a seal member, an O-ring is never advisedly placed in tension in any axis. A little tension is advised only when an O-ring is used as a drive belt or as a rubber-band.
U.S. Pat. No. 3,397,928, issued Aug. 20, 1968 to Galle seemed to violate a standard O-ring handbook rule on the amount of compression that may be employed in a dynamic seal design. The handbook limit even at very low relative seal to surface velocities was set at 10%, and '928 taught the use of 10% to 20% in a rock bit bearing application. The handbooks had predicted a shortened life expectancy for an O-ring used at such excessive levels of compression, and the predictions were well founded. However, a rock bits typical useful life expectancy is only about 100 hours, well within the shortened life expectancy of the over-compressed O-ring. The rock bit is used to virtual destruction. The purpose of the seal in a rock bit is to hold seal integrity for as long as possible as bearing wear permits ever larger relative excursions between the rotary rock cutter and its supporting journal shaft. To extend the life of such a seal in a rock bit, a way has to be found to maintain seal integrity over ever larger relative excursions of the structural members, near the end of the useful life of the rock bit when the journal bearings are most rapidly wearing out.
Heat is devastating to the elastomer of any packing ring. As the seal serves to close a running clearance which is constantly in flux, internal friction within the elastomer is converted to heat. This heat is relatively difficult to dissipate because the elastomer itself is a poor conductor of heat.
On Oct. 16, 1973, U.S. Pat. No. 3,765,495 issued to Murdock et. al., teaching the use of a packing ring having an oval cross section in a rock bit. This design provides an enlarged dimension to the radial extent of the ring without expanding the axial dimension of the ring, as such an axial expansion would serve to reduce the space allocated to the length of the journal bearing. The enlarged radial dimension enabled the use of an adequate radial compression dimension without exceeding the handbook recommendations on the percentage of that same compression. The use of a lower percentage of compression also served to reduce the internal frictional heating of the elastomer, and the relatively low axial dimension of the ring coupled with the increased surface area of the ring served to aid in the dissipation of that internal heat.
Whereas Galle, in '928, showed dimensions in the specification which allowed the ring to be held stretched over the journal shaft surface by as much as 10% of the inside ring diameter. (In addition to the axial compression of the ring itself by 10% to 20% of the nominal cross section of the ring.) This is another case of the tensile extension discussed above in relation to Swart et. al. '973. Murdock, '495 was the first to properly address this problem of tensile extension.
Murdock, '495, uses the conventional clearance on the inactive axial direction to provide space for tolerance buildup and thermal expansion of the elastomeric ring.
A very small running clearance is provided between the sealing surface of the journal shaft and the radially extending side walls of the gland to avoid the potential danger of differential pressures across the seal extruding the elastomer into the running clearance on the low pressure side of the seal.
Murdock suggests a relatively coarse surface within the gland channel, to urge the ring to preferentially rotate with the member forming the channel. Actually, as a practical matter, the ring naturally alternates between clinging to the channel member and clinging to the plain smooth member. For example, suppose the ring is rotating with the channel initially. Frictional heat caused a slight tightening of the ring on the non-rotating shaft until friction with that sealing surface overcomes the roughened grip with the channel, then the channel rotates against the static ring. This condition persists until wear of the ring against the roughened channel surfaces reduces the friction therebetween, and perhaps frictional heat has been dissipated to loosen the grip slightly between the ring and the smooth seal surface, and the ring again rotates with the channel, etc. It should be here noted that, although the wear is actually distributed cyclically over differing surfaces of the packing ring, a roughened surface on one member would serve to increase the rate of wear during one part of the cycle, and would present a poor dynamic seal in cooperation with the packing ring during the remaining part of the cycle.
Except for those rock bits using the seal design of Murdock et. al. rock bits employing packing ring seals do not utilize a channel at all, but depend rather on a "corner gland", wherein the elastomeric ring is trapped between a right corner formed by the juncture of the supporting journal shaft and its supporting leg, and a second reverse corner occurring at the juncture of the internal surfaces of a counterbore formed at the mouth of the journal bore in the cutter cone. Use of the corner gland serves to aggravate two problems which sometimes occur in packing ring seals, "snaking", and "bunching", both being due to an excess amount of gland side clearance being provided for tolerance accumulation and thermal expansion.
"Snaking" describes a condition in which the seal ring, which is normally static relative to one structural member and simultaneously dynamic relative to the other structural member, will switch modes at some local point on the ring circumference. This causes a disarray of forces circumferentially about the ring characterized by the ring piling up on alternating sides of the gland thermal clearance, somewhat snakelike. Then the ring is alternatively squeezed more or less along its length. This instability tends to be self sustaining, is deleterious to both seal ring and gland, and can serve to pump environmental material in one direction or the other across the seal.
"Bunching" starts in the same way as snaking, but the piling up stays concentrated at the point or points of initiation, causes serious leaking, and is apt to break the seal ring itself.
Another type of problem associated with O-rings, called "Rolling", must have been named before it was really understood. Unvarying evidence indicates that rolling is not a dynamic problem as are snaking and bunching. Rolling is a condition established by carelessness in assembly, wherein the O-ring is installed twisted so that the moldline flash spirals about the ring over its circumference. It has been shown that a ring so installed does not change its orientation during use. The danger presented by rolling is that the spiral flash may provide a leak path past the seal, particularly when first placed in service, while the moldline flash is most prominent.
Other innovative packing ring seal designs intended for heavy duty have failed to acquire wide marketability for some reason or another. Examples from the prior art include:
______________________________________ Robinson U.S. Pat. No. 3,656,764 Nielson U.S. Pat. No. 3,944,306 Shields U.S. Pat. No. 4,168,868 ______________________________________